

転んで、思わず地面に膝をついた。
子供と話す時は、膝をついて目の高さを合わせます。
床に膝をつくと痛いので、クッションを使いましょう。
"ひざをつく(膝をつく)" is in the lesson "Emotions" (Level 19), along with 10 other useful words. LinDuo has 180 topic lessons like this one, 2,379 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.